When powering an electronic system, close attention must be paid to the power source or sources and how best to deliver power from the power sources to the electronic system. Almost all power sources have a limited capacity to supply power to a load. A power source has limitations determined by its generator and a transmission line connecting the generator to the load, or electronic system.
Typically, an electronic system is designed having a power specification. A power system is designed that meets the needs of the power specification of the system. In designing the power system, the transmission line between the load and the generator is sized such that power is transferred with an acceptable level of loss and that sufficient power reaches the load. The generator is specified to be capable of supplying the load power plus the transmission line losses.
In the case of telecommunication systems, such as the public switched telephone network (PSTN), there exists a network of wires that can serve as transmission lines to supply power for remotely located electronic systems. Typically, a single power source is used to supply power over a single pair of wires to one or more loads. The remotely located electronic system is commonly referred to as being “span-powered” since direct current (DC) power as well as telephony signals are fed over the pair of wires. Signal repeaters are often employed along the length of the wire pair to maintain power and signal integrity. If the electronic system requires more power than can be provided, even at 100% efficiency, then it becomes necessary to replace the power source with one having a greater capacity. Replacing the power source can be extremely expensive. In some cases it may not be possible to replace the power source.
One piece of telecommunication equipment being used with increasing frequency is digital subscriber line access multiplexers (DSLAMs). DSLAMs make possible high speed data communications, such as the various types of digital subscriber lines (xDSL) communications, to a subscriber such as a home or business computer user. Because of the high power requirements of DSLAMs, they are placed in a central office (CO) or other environment where power is readily and abundantly available. Due to this placement requirement, which is dictated by the power requirements of the DSLAM, an estimated 40 to 80 million people in North America are unable to receive xDSL service. Many of those unable to receive xDSL service would be able to do so if the DSLAM could be remotely located, and remotely powered. However, there is currently no way to meet the power requirements of a DSLAM or other similar piece of equipment placed in a remote location.
One method used in some systems to deliver higher current, and to increase reliability of the power system, uses parallel power converters. Typically, multiple power converters are connected to a single power source with each power converter supplying current to the load by connecting the outputs of each power converter together, also known as power OR'ing. Reliability is increased since if one power converter fails, there are redundant parallel power converters that continue to supply power to the load. Power OR'ing is frequently used in computer systems to supply power to a microprocessor. In such systems there is one power source and the transmission lines from the power source to the power system have no significant resistance, and therefore do not contribute to any significant power loss.
Another method commonly implemented aggregates power from multiple DC power sources such as solar cells. Typically, DC current from multiple solar panels is converted to alternating current (AC) power through the use of multiple inverters. Multiple inverters are synchronized with each other, and with the AC power grid, to combine the outputs of multiple solar panels. The AC power grid aggregates, transmits, and distributes large amounts of power in the form of alternating current. Although the solar cells have power limitations, as in the case of power OR'ing above, the transmission lines from the power sources, that is the solar cells, to the power system have a very small resistance. Furthermore, the output voltage of the solar cells is generally known and is substantially equal from cell to cell. Additionally, the AC power grid is so much greater than the solar system's output in terms of its power capability that reliability and power delivery to the loads is dependent on the grid rather than the DC power sources, or solar cells, which merely contribute energy to the grid.
The methods described above are not applicable to solving the problem of powering a remotely located DSLAM as described above. A single wire pair of the PSTN cannot supply the power required by the DSLAM. Additionally, due in part to the characteristics of the generators and transmission lines of the PSTN, the methods described above that make use of multiple power converters connected to multiple wire pairs will not produce the stable, regulated DC power needed by a DSLAM.
In the PSTN, the characteristics of the transmission lines are unknown and can vary greatly from line to line. Some lines may be extremely short and have little resistance, while other lines may be extremely long and have an extremely large line resistance of on the order of several kohms. Additionally, the voltage of the generators cannot be known exactly. Accordingly, the use of parallel power converters of the prior art in order to aggregate DC power will cause excessive power loss on some transmission lines, and excessive current drain on other transmission lines, which in turn can result in excessive power loss. This behavior causes unregulated and oscillatory behavior at the outputs of the power converters.
Thus, a need presently exists for a method and apparatus for aggregating power and producing a single, regulated direct current voltage from multiple power sources having unknown characteristics.